It is estimated that more than 6,000 platforms have been installed worldwide.
International regulations are now tending to require them to be dismantled at the end of their period of use.
Although on-site dismantling may be considered for small substructures, it is not applicable in practice to substructures such as the large jackets in the North Sea, which may have a bottom section of 100 meters (m) by 80 m, a height of 120 m to 170 m, and a mass of about 30,000 tons.
Publication U.S. Pat. No. 3,347,052 describes a technique for for recovering an offshore substructure which consists in fixing permanently or temporarily a certain number of floats to the legs of the structure at a judicious selection of locations and in selectively ballasting the floats to change the position of the structure from its in-service vertical position to a horizontal position where the structure is floating and is capable of being towed.
Installing floats at accurate locations and the synchronization required for their ballasting constitute operations that are difficult, and it is also difficult to perform the essential monitoring of the positions and the movements of the floating structure.
It is also known that a substructure may be built on a barge that can be ballasted, the barge being used to convey the substructure to the location where it is to be installed, and being suitable by selective ballasting for tilting so as to move the structure from its horizontal transport position to its vertical in-service position (Publication U.S. Pat. Nos. 3,823,564 and 3,987,637), however this technique also requires the substructure itself to be selectively ballasted and it is not suitable for recovering a substructure.
An object of the present invention is to enable a substructure which is generally in the form of a truncated pyramid to be recovered by means of a ballastable barge without requiring the substructure itself to be ballasted and unballasted.